From Ice up to ice out

Image
  • Outdoors
    Outdoors
Body

By Vicki Browne
Iron County Lakes & Streams Partnership
The Feb. 22 edition of the Reporter contained an article about deteriorating ice – “Deteriorating ice may prompt early shanty removal”. The article made me want to know more about how lake ice develops and eventually deteriorates.
Two gentlemen with lots of ice experience provided that information through their web posts: Bob Dill, an engineer whose website explains ice dynamics (lakeice.squarespace.com) and John Downing, a limnologist and director of the Minnesota Sea Grant Program.
Downing’s March 1 post about predicting when ice-out will occur will be particularly interesting to citizen scientists who enjoy gathering data and modeling trends.
In the summer, deep lakes have a top layer that is warmer, a middle layer where the temperature steadily decreases, and a bottom layer that is colder. In the fall, the top layer cools, becomes denser and sinks toward the bottom. This progression of cooling and sinking continues until the entire lake is about 39°F.
As the temperature of the surface water continues to cool beyond 39°F, the water stops becoming denser and actually becomes less dense. The water molecules spread out and form a regular pattern that will eventually become ice. Ice starts at the top of the lake and floats there because it is 9% less dense than water.
‘Fast ice’ is a term used for the ice that completely covers a lake from shore to shore. Fast ice is held fast by the shore. The two major factors in the development of this initial layer of ice are temperature and turbulence.
If nights are calm and temperatures are below freezing, a smooth sheet of primary ice will spread across a lake. This ice is called black or blue ice.
If winds are strong and temperatures are below freezing, fine ice crystals shaped like sharp points or small disks will float in the water. These clusters of crystals are called ‘frazil’. Under turbulent conditions, the ice crystals will also form slush and sludge. When turbulence defines the development of the primary fast ice, that ice is called white ice. It is weaker than black ice.
Once a primary layer of ice catches and covers a lake, that ice will continue to thicken. If the weather is cold but variable, the secondary ice will be cloudy. If the weather is consistently cold for a long period, the secondary ice will be clear.
When the ice is thin and not covered with snow, it thickens more quickly. This is because heat can still escape. As ice is thickening, a person can predict how fast this will occur by tracking average temperature over a 24-hour period. Subtract this temperature from 32°F to calculate the ‘freezing degree days’ (FDD). This isn’t really a number of ‘days’, but rather a unit of measure.
For example, if the day’s average temperature is 12°F, then 32 - 12 = 20 FDD units. For each 15 FDD units, lake ice thickens by approximately one inch. So on this day the ice would gain about 1 1/3 inches of thickness. Less ice forms once snow begins to insulate the primary fast ice.
Superimposed ice is the slushy ice that forms on top of the primary layer of fast ice. It comes from rain, runoff and melted surface snow. Superimposed ice often freezes on top but leaves slush and water between its top and the primary ice layer.
At this time of year, we’re thinking more about ‘thawing degree day’ units (TDD). These aren’t actually ‘days’ either. To compute TDD units, subtract 32°F from the day’s average temperature. For example, if the day’s average temperature is 40°F, then 40–32=8 TDD units.
According to John Downing’s recent article, about half the lakes experience ice out after there have been a total of 220 TDD units. Of course, snow-covered lakes need a lot more than 220 TDD units to get to ice out. A good site to collect the day’s average temperature is the National Weather Service out of Stambaugh (weather.gov/wrh/Climate?wfo=mqt).
Thawing is actually ice decaying. As ice begins to decay, the strength of the ice is more important than its thickness when considering whether to walk or drive on it.
As spring approaches, the sun is almost twice as intense as during the winter. Warmer temperatures, high winds and underwater currents all cause the ice to decay. Warm winds cause the top surface to melt. Intense sun weakens the internal structure of the ice, sometimes transforming it into vertical ‘candles’ which are very unstable. The ice also experiences fatigue along routes that were driven by vehicles. And where pressure ridges formed in the ice, the wind will be able to push ice over onto itself or the shore.
As the insulating snow cover melts or blows away, the primary ice layer will be revealed once again. The ice’s decay will have its own symphony of sounds — zings, creaks, groans, moans, pops, rifle shot and whale songs. As it warms and shrinks from the shore and then cools to expand and push against the shore, it will also crack and shove over itself. And eventually, on a windy, warm day, it will be a lake again.